Storyline 

After an initial unsuccessful attempt to defeat Aquaman, Black Manta remains determined to avenge his father's passing. His pursuit of justice has only intensified, making him more dangerous than ever. Now equipped with the mythical Black Trident, he commands a powerful and sinister force from the past. In order to vanquish him, Aquaman must seek help from an unexpected ally—his incarcerated brother Orm, the previous ruler of Atlantis. They must put their conflicts aside and unite in their efforts to safeguard their kingdom, Aquaman's family, and indeed, the entire world from impending catastrophic devastation.

Plot

Arthur Curry, several years into his reign as King of Atlantis, leads a dual life on land and sea, balancing his roles as a husband to Mera, a father to their son Arthur Jr., and a king. Meanwhile, David Kane, still nursing a grudge against Arthur for his father's demise, collaborates with marine biologist Stephen Shin to unearth Atlantean artifacts. He discovers a potent black trident, which possesses him, guaranteeing the power he craves to defeat Arthur. Five months later, David invades Atlantis, utilizing its orichalcum reserves to fuel Shin's Atlantean machinery. Arthur discovers the misuse of orichalcum, a material that releases copious greenhouse gases, has escalated global temperatures, triggered extreme weather, and induced ocean acidification. Moreover, it nearly brought about a planetary extinction when an ancient Atlantean kingdom exploited it. Determined to locate David, Arthur liberates his half-brother Orm from prison and, together, they align with crime lord Kingfish. This alliance guides them to a volcanic island in the South Pacific. On the island, Arthur and Orm encounter the black trident. Orm reveals its creation by Kordax, King Atlan's brother and ruler of the lost kingdom of Necrus who was incarcerated with blood magic after a botched coup. Realizing that the blood of Atlan's descendants could liberate Kordax, they head to Amnesty Bay, only to discover that David has abducted Arthur Jr. With Shin's reluctant assistance, the Atlanteans deduce Necrus's prison is situated in Antarctica. In Necrus, a battle ensues between Arthur and David to protect Arthur Jr. David nearly kills Arthur, but Mera intervenes, saving her husband. In the ensuing chaos, David hurls the black trident at Mera, but it is intercepted by Orm. The spirit of Kordax abandons David for Orm, leading to an intense duel between Arthur and Orm. Orm's blood thaws the ice, liberating Kordax. Arthur retaliates by hurling the black trident at Kordax, who catches it mid-air, only for Arthur to throw his trident, shattering the black one and fatally wounding Kordax. With Kordax's magic extinguished, Necrus starts to disintegrate. As David teeters on the edge of a crevice, he rejects Arthur's aid, instead plunging to his death. The Atlanteans and Shin escape, while Arthur and Mera release Orm on the condition that he remains hidden and reports his own death to Atlantis. Recognizing the need for unity between land and sea, Arthur announces the existence of Atlantis at the United Nations, declaring his intention to make the kingdom a member state.